Conventionally, a lithographic printing plate has been manufactured in a system of exposing the printing plate precursor through a lith film as an intermediate material. However, with recent rapid progress of digitization in the printing field, the system for the manufacture of a printing plate is changing into a CTP system where digital data input and edited in a computer is directly output on a printing plate precursor. Among these techniques, with an attempt to more streamline the process, a lithographic printing plate precursor which can be fixed on a press as it is after exposure without passing through a development processing and used for printing is being studied and developed. Various methods for obtaining a CTP printing plate capable of dispensing with development are described, for example, in Nippon Insatsu Gakkai Shi (Journal of Japan Printing Society), Vol. 36, pp. 148-163 (1999).
As one of the methods for dispensing with the processing step, a method called on-press development is known, where an exposed printing plate precursor is fixed on a plate cylinder of a press, and a fountain solution and an ink are supplied while rotating the plate cylinder, thereby removing the non-image area of the image-recording layer of the printing plate precursor. Namely, this is a system of fixing a printing plate precursor as it is on a press after exposure and completing the development processing during the normal operation of initiating the printing. The lithographic printing plate precursor suitable for such on-press development is required to have an image-recording layer soluble in a fountain solution or an ink solvent and moreover, to have a bright room handling aptitude of not causing fogging due to visible light even if developed on a press installed in a bright room.
For example, Japanese Patent 2,938,397 describes a lithographic printing plate precursor where a photosensitive layer comprising a hydrophilic resin having dispersed therein thermoplastic hydrophobic polymer fine particles is provided on a hydrophilic support. In this patent publication, it is stated that the on-press development can be performed by exposing the lithographic printing plate precursor with an infrared laser to cause combination (fusion) of the thermoplastic hydrophobic polymer fine particles due to heat and thereby form an image, then fixing the plate on a plate cylinder of a press, and supplying a fountain solution and/or an ink. This lithographic printing plate precursor also has bright room handling aptitude because the photosensitive region thereof is in the infrared region. However, such a lithographic printing plate precursor having an image-recording layer comprising a hydrophilic binder resin having dispersed therein hydrophobic polymer fine particles has a problem in that when exposed with an infrared laser having high energy, in addition to the image formation by the combination of fine particles, the image-recording layer partially undergoes ablation and the quality as a printing plate deteriorates.
To solve this problem, EP-816070 describes a technique where an image-recording layer comprising a hydrophilic binder having dispersed therein a hydrophobic thermoplastic polymer particle and a light-to-heat (photothermal) converting agent is provided on a hydrophilic support and further thereon, a water-soluble or water-swellable protective layer comprising a hydrophilic resin is provided to prevent the ablation.
Also, WO98/51496 describes a lithographic printing plate precursor which is exposed, developed with an aqueous alkali solution or the like and then fixed on a press, where the ablation can be effectively prevented by providing two image-recording layers each comprising an aqueous solution-soluble or swellable binder having dispersed therein fine particles, and setting the optical density of the upper layer at the exposure wavelength to be lower than that of the lower layer.
The lithographic printing plate precursor according to Japanese Patent 2,938,397 has a problem in that at the time of coating and drying the image-recording layer, the resin fine particles are fused to cause fogging. If the drying is performed at a low temperature over a long time so as to prevent the fusion of resin fine particles at the coating and drying, the production efficiency decreases and this means is not practicable. Also, means of using a particle adhesion inhibitor such as water-soluble resin disadvantageously causes deterioration in the inking property. JP-A-2000-141933 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) describes an image-forming material capable of on-press development, which has a layer containing high molecular polymer fine particles having two or more peaks in the particle size distribution, and states that those problems can be solved by this material.
JP-A-2000-221667 describes an image-forming material capable of on-press development, which has an image-recording layer containing two or more kinds of polymer fine particles different in the minimum film formation temperature, and states that the problems in the image strength, deterioration of impression capability and stable supply of fountain solution, encountered in conventional on-press lithographic printing plate precursors, can be overcome, as a result, a stable printing quality can be obtained.
JP-A-9-127683 describes a printing plate which can be produced by the on-press development using a self water-dispersible resin particle. This printing plate is advantageous in that since the non-fused resin particle has high hydrophilicity, the resin particle in the non-image area readily releases from the substrate surface and the non-image area is reduced in the ink staining.